Optical disk recording and playback devices use Focus/Tracking actuators to control the lens position relative to the rotating disk. This is necessary because of unavoidable warp of the disk, spindle errors, and other mechanical and optical imperfections. Both focus (controlled by the distance between the lens and the disk surface) and tracking (the correct radial position of the lens so as to read or write a single track at a time) must be simultaneously maintained for the reader or recorder to function. The motions of the actuator are controlled by a servo system and the forces which cause these motions are typically generated by one or more electromagnetic motors.
As optical disk recording and reading devices increase in speed (read and write rate), the disk rotational velocity increases. This requires that the Focus/Tracking actuator, which controls the objective lens position so as to maintain proper focus and tracking location of the read/write spot on the disk surface, must respond more quickly and accurately to any disk warping or spindle errors. This in turn requires that any internal actuator resonances be either sufficiently high in frequency, or have sufficient damping so as to allow the required control system performance. Often, the flexures, in flexure-supported actuators, have problematic resonances with low damping, which limit the actuator servo performance.
FIG. 1 shows a conventional optical actuator generally designated 10. The optical actuator 10 includes an objective lens 12 which is well known in the art to focus a laser light beam (not shown) on the surface of an optical disk (not shown). The objective lens 12 is fixed to a lens holder 16 by an adhesive or other fastening structure (not shown). The optical actuator 10 further includes four spaced-apart flexures 18. The flexures 18 are fixed at one end to the lens holder 16 and at their other ends to a base 36. All of the flexure ends are fixed by rigid fixing means; that is, the ends of the flexures are constrained to the base 36 or the lens holder 16 in all six degrees of freedom of motion. An actuator focus coil 38 and tracking coils 40 are bonded to the lens holder 16. These coils are positioned within the magnetic flux generated by magnets 32 and magnetic flux yokes 34. The focus coil 38 and tracking coils 40 provide forces which deform the flexures 18 and move the lens holder 16 and attached objective lens 12 in the Y and Z directions so as to maintain the focus and tracking of the laser beam (not shown) on the optical disk (not shown). Because the flexures 18 are typically manufactured of metal wire, which has very low internal damping, and the end fixing means of the flexures 18 do not add significant damping, the resonances of the wire flexures have very little damping. These resonances are typically in the frequency range of 500 to 10,000 Hz, and cause problems with the stability and performance of the servo used to control the actuator.
Further, as storage capacity is increased, for example in the DVD disk, higher numerical aperture lenses are required for recording and reading data on the disk. Conventional two degrees of freedom actuators (focus and tracking) do not compensate for disk tilt and therefore their performance suffers with use of high numerical aperture lenses. Accordingly, there is a need for an improved four degree of freedom actuator (focus, tracking, pitch and roll) for use with high capacity storage media.